The importance of cortical granules in the block to multiple sperm penetration in eggs of the toad and mouse is being investigated by biochemical and morphological techniques. The temporal interrelationships between cortical granule dehiscence, sperm penetration of the ovum and fertilization-associated changes in the mouse zona pellucida have been evaluated. Partial release of granule contents precedes sperm-egg fusion in eggs inseminated in vitro. The triggering event in this process is being pursued with sperm supernatants as primary candidates. The role of the zona and plasmalemma in limiting capacitated or fresh sperm penetration of the mouse ovum will be approached by inseminating zona-intact and zona-free eggs. Cortical granules will be isolated from eggs of the toad, and their contents will be compared with granule exudate recovered from artificially activated, intact eggs. The role of the vitelline-fertilization envelope in early development of Xenopus will be assessed by removing or chemically altering the envelope before or immediately after fertilization.